1. Field of the Invention
The present invention relates to an optoelectronic device and more particularly, to an optoelectronic device having an optical element such as a laser diode, photodiode mounted on a body with leads.
2. Description of the Prior Art
A conventional optoelectronic device of this sort is shown in FIGS. 1A and 1B.
As shown in FIG. 1A, the conventional optoelectronic device has a metal stem composed of a circular metal plate 21 as a base and a metal block 34 as an element mount plate on the top surface of the plate 21. The block 34 is fixed on the metal plate 21 or is integrated with the plate 21. The block 34 has a horizontal cross-section of an arc and a flat element-mount face 34a at one side thereof.
The metal plate 21 has four metal leads 22, 23, 24 and 25 at the bottom surface thereof which are arranged along the periphery of the plate 21 at almost regular intervals. The top ends of the leads 22, 23, 24 and 25 are fixed to the plate 21, respectively. All the leads 22, 23, 24 and 25 are of columnar shapes.
The top end of the lead 25 is electrically connected to the metal plate 21 and does not protrude from the top surface of the plate 21. The top ends of the leads 22, 23 and 24 are electrically insulated from the metal plate 21 and slightly pass through the plate 21 to protrude from the top surface thereof. As shown in FIG. 1A, the top end of the lead 24 is formed to a flat, thin plate 24a.
A semiconductor laser 27 as a light-emitting element is adhered to be fixed on a heat sink 26 which is fixed to the flat element-mount face 34a of the block 34. The heat sink 26 is made of insulating material and the mounting surface thereof is covered with a metal layer electrically connected to the block 34 by a piece 31 of bonding wire. The laser 27 is mounted on the mounting surface of the heat sink 26.
The anode of the laser 27 is in contact with the metal layer of the heat sink 26 so that it is electrically connected to the metal plate 21 and the lead 25 through the piece 31 of bonding wire and the metal block 34. The cathode of the laser 27 is electrically connected to the protruded top end 24a of the lead 24 by a piece 30 of bonding wire.
A photodiode 29 as a light-receiving element is adhered to be fixed on a rectangular alumina plate 28. The alumina plate 28 is fixed on the top surface of the metal plate 21 through an insulator. The photodiode 29 receives to monitor the light emitted from the laser 27.
The cathode of the photodiode 29 is in contact with the alumina plate 28 and the plate 28 is electrically connected to the top end of the lead 22 by a piece 32 of bonding wire, and as a result, the cathode is electrically connected to the lead 22. The anode of the photodiode 29 is electrically connected to the protruded to pend of the lead 23 by a piece 33 of bonding wire. The photodiode 29 itself is electrically insulated from the metal plate 21.
The electrical connections of the optoelectronic device described above is shown in FIG. 1B. The semiconductor laser 27 is driven by a dc voltage applied across the leads 24 and 25 and the photodiode 29 is driven by a driver circuit (not shown) connected to the leads 22 and 23.
Another conventional optoelectronic device of this sort is shown in FIGS. 2A and 2B.
The optoelectronic device shown in FIG. 2A is the same in structure as that shown in FIG. 1A excepting that a piece 31a of bonding wire is provided for electrically connecting the top end of the lead 22 and the heat sink 26 instead of the piece 31 of bonding wire bonded to the metal block 34 and the heat sink 26.
Therefore, the anode of the semiconductor laser 29 is electrically connected to the cathode of the photodiode 29 by the piece 31a of bonding wire. The anode of the semiconductor laser 29 is electrically insulated from the metal plate 21.
This optoelectronic device has electrical connections as shown in FIG. 2B.
The market mostly has been demanding the electrical connections in FIG. 1B, however, recently, it is demanding those in FIG. 2B increasingly.
With the conventional optoelectronic device shown in FIG. 2B, the electrical connections are performed by the following bonding sequence:
First, an end of a continuous bonding wire is bonded to the top end of the lead 23 by a wire bonding machine, and the continuous bonding wire is then bonded to the anode of the photodiode 29 to be cut. Thus, the top end of the lead 23 is linked with the anode of the photodiode 29 by the piece 33 of bonding wire.
Next, the end of the continuous bonding wire is bonded to the alumina plate 28, and the continuous bonding wire is then bonded to the top end of the lead 22. Thus, the cathode of the photodiode 29 is linked with the top end of the lead 22 by the piece 32 of bonding wire.
The continuous bonding wire is further bonded to the heat sink 26 fixed to the element-mounting surface 34a of the block 34 to be cut. Thus, the top end of the lead 22 is linked with the anode of the semiconductor laser 27 by the piece 31a of bonding wire.
Finally, the end of the continuous bonding wire is bonded to the anode of the semiconductor laser 27, and the continuous bonding wire is then bonded to the top end 24a of the lead 24 to be cut. Thus, the cathode of the laser 27 is linked with the top end 24a of the lead 24 by the piece 30 of bonding wire.
In this bonding sequence, there is a problem that the bonding head of the wire bonding machine and the plate 21 are required to be turned by 90 degrees in the step of forming the piece 31a of bonding wire, respectively.
In this step, the bonding face or the anode surface of the semiconductor laser 27 is oriented horizontally and the bonding face or the top end face of the lead 22 is oriented upward. Therefore, these two bonding faces are perpendicular to each other. Also, the positions of the bonding faces thereof are apart from each other along the surface of the plate 21.
As a result, the bonding head oriented downward in the bonding step of the lead 22 is required to be turned by 90 degrees to be oriented horizontally, and at the same time, the plate 21 is turned by 90 degrees around its central axis so that the bonding face of the laser 27 is placed at a given bonding position of the wire bonding machine in the step of forming the piece 31a of bonding wire.
Generally, when the wire bonding processes are performed between the perpendicular surfaces, the obtainable bonding strength is remarkably reduced compared with the case that the processes are performed between the parallel surfaces, and as a result, it is difficult for sufficient bonding reliability to be ensured, resulting in less reliability of the optoelectronic device shown in FIGS. 2A and 2B.
Additionally, there is another problem that the working table of the wire bonding machine has to be equipped with a turning mechanism during a bonding process, which produces increase in cost.
To solve the above problems, a conventional optoelectronic device as shown in FIG. 3 has been developed, which was disclosed in the JAPANESE NON-EXAMINED PATENT PUBLICATION NO. 2-86184.
As shown in FIG. 3, on a circular metal plate 41 as a base, there is a rectangular parallelepiped metal block 54 as an element mount. A semiconductor laser 47 as a light-emitting element is adhered to be fixed on a flat side face of the block 54. The block 54 also acts as a heat sink for the laser 47.
The metal plate 41 has three metal leads 42, 43 and 45 arranged at the bottom surface thereof at intervals. The top ends of the leads 42, 43 and 45 are fixed to the plate 41 electrically insulated from the plate 41 by glass films 60, respectively. All the leads 42, 43 and 45 are of columnar shapes.
The top end of the lead 45 does not protrude from the top surface of the plate 41. The top ends of the leads 42 and 43 slightly pass through the plate 41 to protrude from the top surface thereof. Bonding faces 42a and 43a are produced on the top end faces of the leads 42 and 43, respectively.
There is a rectangular parallelepiped insulator block 62 adjacent to the metal block 54 on the top surface of the plate 41. The insulator block 62 is made of ceramic such as alumina (Al.sub.2 O.sub.3), SiC, ZiO.sub.2, Si.sub.3 O.sub.4. A metal layer 63 is formed on the insulator block 62 by a metallization process. The layer 63 extends from a top face of the block 62 to a side face thereof.
The flat top face of the insulator block 62 is substantially the same in height as the flat top face of the metal block 54, and both of the top faces are parallel to each other. The flat side face metallized of the insulator block 62 and the anode and cathode surfaces of the laser 47 are parallel to each other.
The anode surface of the laser 47 is in contact with the side face of the metal block 54 to be electrically connected to the metal block 54, and the cathode surface thereof is electrically connected to the metal layer 63 at the side face of the insulator block 62 by a piece 51 of bonding wire. The metal layer 63 is electrically connected to the bonding face 42a of the lead 42 at the side face of the insulator block 62 by a piece 64 of bonding wire.
A photodiode 49 as a light-receiving element is adhered to be fixed on the top surface of the plate 41 adjacent to the metal block 54. The cathode surface of the photodiode 49 is in contact with the top surface of the metal plate 41 to be electrically connected to the plate 41, and the anode surface thereof is electrically connected to the bonding face 43a of the lead 43 by a piece 53 of bonding wire.
The electrical connections of the optoelectronic device in FIG. 3 is the substantially same as that shown in FIG. 2B.
With the optoelectronic device in FIG. 3, although the above problems can be solved, there arises a different problem that the metal layer 63 needs to be formed and patterned on the insulator block 62 and that the insulator block 62 thus metallized needs to be fixed on the metal plate 41.
This means that the number of the assembly process steps is increased and the optoelectronic device thus assembled is more complex in structure compared with the conventional ones in FIGS. 2A and 3A, producing increase in cost.